This world has been the home to the majority of humanity since the Cataclysm on Earth, which has only just surpassed it in terms of population. Approximately 6 billion live on this mostly terraformed world, out in the open in it’s created and boosted atmosphere. Mars is largely responsible for the spread of humanity to the rest of the solar system. This was done after mankind has acclimated and evolved (somewhat) to Mars’ lesser gravity, and as a result most of humanity is unable to return to its original homeland of Earth for very long, and even then not without artificial aids.